Field effect transistors (FETs) used in SRAMs and analog circuits (e.g., differential amplifiers, DACs, ADCs) need to have their VTsat and VTlin measured with high precision (<1 mV range) and high speed to characterize/quantify components (devices) on the wafers. The VTsat and VTlin need to be quantified absolutely and relatively between devices to allow a customer to match devices used in e.g., DACs, ADCs, etc.
High precision target is set by mismatch impact on several analog circuits' performance degradation sensitivity. High speed is needed so that as many as possible devices can be measured to determine reliable statistical values in a short time period. This is strongly driven, for instance, by SRAMs.
VTsat and VTlin are measured with static IC test equipment. By way of example, defined drain current Id is forced into the circuit and gate source voltage Vgs has to be set so that defined drain source voltage Vds results. This force and sampling procedure is executed iteratively until Vgs (i.e., VTsat and VTlin) is found for a Vds which is close enough to the defined value. A tradeoff between the number of iterations (test time) and achieved precision of VTsat and VTlin precision has to be made, though. That is, there is a tradeoff between precision measurements and speed of measurements. For example, static test equipment needs sampling times for analog signals in the one to two digit millisecond range.